Once the tables have been constituted, it is indispensable for the correct operation of the decoder that it acquires an internal reference clock. The acquisition is carried out by recovering the clock information item most often via the source constituted of the flux of signals received by the decoder. Once the clock information item has been received, it constitutes the internal reference clock of the decoder, while being advantageously updated from time to time by the same source as that which had allowed its initial acquisition.
Such a reference clock is used permanently in decoding operations, but also in ancillary operations such as for example the purchase of video on demand (VOD). It makes it possible in particular to limit the use of VOD, once it is purchased, to a certain duration limited in time, usually by comparing the internal reference clock of the decoder with a clock information item that is available on a remote server. Certain persons guilty of fraud manage however to intervene on the internal reference clock of the decoder in order to bypass the security of VOD, consisting in limiting the viewing thereof to a limited time. It thus appears in this precise case of modifying the principle of the internal reference clock such as it has existed until now.
In a more general manner, for various applications of digital television decoders, and particularly for applications for controlling the correct operation of said decoders, it henceforth appears insufficient, the reference clock information item having become fundamental in the operation of the digital television decoder, to limit oneself to the normal operation until now; this consisted in storing a unique clock information item acquired via a unique source to determine said internal reference clock information item of the decoder.